Graphene is a material having a two-dimensional planar structure in a honeycomb shape, in which carbon atoms are connected to one another to form a hexagonal structure, and exhibits chemically high stability. In addition, since graphene has at least one hundred (100) times greater electrical conductivity than silicon and is flexible and transparent, it is drawing much attention as a next-generation semiconductor material.
With respect to a method for producing a quantum dot of graphene, there are known a top down method that sizes down large-size graphene, a bottom up method that self-assembles and then pyrolyzes a small carbon ring material like hexa-peri-hexabenzocoronene (HBC), and others. In addition, Korean Patent Application Publication No. 2013-0050167 discloses a method for producing a graphene quantum dot and a graphene quantum dot produced by the method. However, these methods have a limit in large-scale production of graphene quantum dots. Further, since a graphene quantum dot is known to have applicability in various fields, it is necessary to develop a technology capable of producing high crystalline graphene quantum dots in a large scale for research of physical properties and application of the graphene quantum dots.
Graphene quantum dots (GQDs), which are graphene sheets that are smaller than 100 nm, possess strong edge effects and quantum confinement. The edge effects allow dispersion in solvents such as ethanol, while graphene, which is a pure carbon material, is not dispersible in common solvents. Graphene is a zero band gap semiconductor, which means its electronic and optoelectronic properties are reduced and it is almost impossible to use it for device applications. However, quantum confinement allows the band gap of GQDs to be controlled by modulating their size. GQDs can exhibit photoluminescence due to their band gap. Their dispersible property, nonzero band gap, chemical inertness, biocompatibility, low toxicity and strong photoluminescence make them excellent materials for applications such as nanoscale optics, electronic devices, bioimaging, OLEDs, fuel cells, photovoltaic devices, composites and biosensors.
Controlled fabrication methods for stable graphene nanostructures provide a chance to investigate outstanding optical and transport properties of these structures. Both top-down and bottom-up methods have been used to prepare GQDs. Cutting of graphene sheets or graphene oxide sheets or carbon fibers or self-assembled block copolymer or tattered graphite or carbon black or coal corresponds to a top-down method, while self-assembling of aromatic carbons followed by pyrolysis, cyclodehydrogenation of polyphenylene precursors, microwave-assisted hydrothermal method, tuning the carbonization degree of citric acid, and pulsed laser synthesis method from benzene correspond to a bottom-up method. Cage-opening of fullerenes may be categorized as a third method. However, these methods have some drawbacks in the aspects of low-cost production, size-controllable fabrication, and mass production.